1. Field of the Invention
The present invention relates to digital image processing and, more particularly, to processing continuous-tone images into halftone images.
2. Description of the Related Art
Digital Halftoning is the process of transforming a continuous-tone image into a binary image that has the illusion of the original continuous-tone image. See, R. Ulichney, Digital Halftoning, MIT Press, Cambridge, Mass., 1987. In the case of color images, the color continuous-tone image is typically separated into color channels first. Separate halftones are then formed for each of the color channels.
Image Resolution Enhancement Technology (IRET) utilizes a mixture of any ordered halftone screen (e.g. clustered-dot dither, line screen, etc.) to generate halftone dots with a number of levels, and any dispersed halftone screen to generate additional levels for the ordered halftone dots. For some printing technologies, it is preferable to minimize printing artifacts by generating coarser halftone screens, like 150 lpi (lines per inch) rather than finer halftone screens like 600 lpi. Coarser screening, however, means that details in the image, text, and line art will not be rendered well. This is especially a problem in copier applications, where the text and line art are scanned rather than computer generated. For these types of applications, a halftoning technique is required that maximizes the artifact reducing properties of coarser ordered screens, while minimizing the loss of rendered detail in image areas having high spatial frequency.
Thus, it can be seen that halftone imaging techniques impose image quality limits upon halftone image output devices, and hinder the use of these devices in many applications.
Therefore, there is an unresolved need for an image resolution enhancement technique that can improve halftone imaging by changing ordered halftone screen resolution according to the content of the image and managing these changes based on image content.